1. Field of the Invention
The present invention relates to an image developing device for electrophotography, and more particularly to an electrophotographic image developing device having various members pressed against other members for developing latent images.
2. Description of the Background
Many electrophotographic image developing devices employing one-component developer (hereinafter referred to as "toner") now in use include various members pressed against other members for developing latent images.
For example, a contact-type image developing device has a toner carrier comprising a developing roller which is held under given pressure against the surface of a latent image carrier in the form of a photosensitive drum or the like for developing a latent image formed on the latent image carrier. In such an image developing device, a toner limiting member such as a thin-toner-layer forming roller is frequently disposed in pressed contact with the surface of the toner carrier for pressing new toner supplied onto the toner carrier into a thin toner layer of uniform thickness. Additionally, a toner supply member comprising a toner supply roller or the like is often positioned in pressed contact with the surface of the toner carrier for scraping remaining toner off the toner carrier after the image has been developed and also for supplying new toner onto the toner carrier.
The toner carrier, the toner supply member, and the toner limiting member are rotated by a rotation transmitting mechanism comprising drive gears directly fixed to the support shafts of these members and held in mesh with each other. In the rotation transmitting mechanism of this type, however, when the device members or components are rotated, driving force components act on the support shafts due to the pressure angles (which are normally about 20.degree.) of the drive gears, and the pressure under which driver members are held against driven members is increased or reduced.
Therefore, the contact pressures between the device members are not uniform, causing the thickness of the thin toner layer transferred into an image developing area to become irregular. As a result, the toner density of a developed visible image is uneven, and the produced copy suffers from background contamination and low resolution. Particularly, contact failure of the members tends to cause a hollow area in a developed image and image density irregularities. Further, excessive contact pressures is apt to develop background contamination due to undesirable toner deposits, and jitter is caused by an increased load.
One solution to the above problems has been to simultaneously rotate the opposite ends of the support shaft of each of the device members. However, this arrangement requires a complex drive mechanism and results in an image developing device of larger size.
Where the toner carrier such as a developing roller and the thin-toner-layer forming member are made of a highly rigid material such as metal, synthetic resin, or the like, and such have fixed axes, these members are liable to be pressed against each other under varying pressures as they rotate because of their accuracy tolerances. As a result, the formed toner layer may be of uneven thickness and may be locally reduced in thickness. It has been proposed to eliminate these drawbacks by constructing the thin-toner-layer forming member as a resilient roll. However, the resilient roll still tends to undergo pressure variations during rotation thereof, making it difficult to stabilize the amount of charging of the toner. Since the resilient roll presents high frictional resistance, it cannot be adequately cleaned. It has also been proposed to plate the surface of the resilient roll with metal to reduce the coefficient of friction of the roll for thereby improving its cleanability. When the resilient roll is elastically deformed under pressure, however, the plated metal layer is liable to come off or be cracked. The metal-plated resilient roll is thus poor in durability.
In view of the aforesaid image developing devices using one-component developer, which has been used or proposed, the inventors have proposed an image developing device of the kind described above which includes a developing roller in the form of a resilient roll such as a silicone rubber roll in which carbon of a hardness ranging from 15.degree. to 50.degree.JISA is dispersed, or an NBR rubber roll with a urethane layer disposed on its peripheral surface, and a thin-toner-layer forming member of metal or synthetic resin which is supported so that its axis is movable with respect to the axis of the developing roller, the thin-toner-layer forming member being pressed under constant pressure against the developing roller by gravity and by a spring.
This arrangement has been successful in eliminating toner layer thickness irregularities and making uniform the amount of charging of the toner, thus making it possible to develop images adequately. This proposed image developing device can be employed for contact-type or noncontact-type image development, but is more advantageous when used for contact-type image development.
FIGS. 10 and 11 of the accompanying drawings illustrate an arrangement of such a contact-type image developing device. A thin-toner-layer forming member or roller 49 is rotatably supported by bearings 52 fitted in guides of a bearing guide member 51 with clearances around the bearings 52 in every direction. The bearings 52 are normally urged toward a developing roller 46 under the bias of springs (not shown). If the developing roller 46 and the thin-toner-layer forming member 49 have an accuracy error, the bearings 52 are moved in the respective guides of the bearing guide member 51 in various directions as indicated by the arrows (FIG. 10) in a plane normal to the axis of the member 49, so that the thin-toner-layer forming member 49 can be pressed against the developing roller 46 under constant pressure at all times.
As shown in FIGS. 12 and 13, the proposed image developing device 42 may be positioned differently with respect to a latent image carrier or photosensitive drum 41. The image developing device 42 may be used in combination with a latent image carrier such as a photosensitive belt 41' as shown in FIG. 14. Those parts in FIGS. 12, 13 and 14 which are identical to those in FIG. 10 are denoted by identical reference numerals.
In the image developing device 42 proposed by the inventors, a toner supply member 45 mechanically scrapes unused or remaining toner off the developing roller 46 and then supplies new toner by way of mechanical frictional transfer to the developing roller 46 as initialized. When the toner is frictionally supplied, it is triboelectrically charged and electrostatically attracted to the developing roller 46. In order to eliminate toner supply irregularities, however, the toner should be supplied more effectively and efficiently from the toner supply member 45 to the developing roller 46.
Particles having a diameter of about 10 micrometers, such as toner particles used as an image developer, can triboelectrically charged. It is known that when particles of idential diameters are triboelectrically charged, they are charged to different polarities. The toner particles charged to opposite polarity increase their charge distribution when an image is developed, thus obstructing uniformity of image development. The oppositely charged toner is responsible for contamination of the background of a copy.